INORGANIC POLYPHOSPHATE AS A MITOCHONDRIAL CHAPERONE IN AGING. Mitochondrial dysfunction plays a crucial role in aging and in neurodegenerative diseases, such as Alzheimer?s disease. One of the main contributors to this dysfunction is impairment of the protein homeostasis systems and the consequent accumulation of misfolded proteins within mitochondria. This protein dyshomeostasis is triggered by pH alterations, heat shock and, especially, by increased reactive oxygen species (ROS). Given that normal mitochondrial function is the main contributor to ROS generation, ROS accumulate to much higher concentrations in mitochondria than in other subcellular compartments. To avoid this dyshomeostasis, organisms have developed the unfolded protein response (UPR), mediated primarily by chaperones and proteases. UPR in the endoplasmic reticulum (ER), (UPRER) has been studied extensively. In contrast, the regulation and function of the mitochondrial UPR, (UPRmt), remains poorly understood, especially in mammals. Our current knowledge about chaperones and proteases cannot explain the robust system that protects mitochondria in young and healthy organisms from the high rates of protein misfolding present in this subcellular environment in aging and neurodegeneration. This suggests that an alternative and powerful mechanism maintains protein homeostasis in mitochondria. Recently, inorganic polyphosphate (polyP), which is a well-conserved molecule among different species, was described as a primordial polymer with a universal chaperone role in bacteria, as well as in other organisms, including mammals. We hypothesize that polyP is an integral component of mitochondrial protein homeostasis. This proposal aims to determine the importance of polyP as a chaperone in mammalian mitochondria, suppressing protein dyshomeostasis and maintaining the correct balance of mitochondrial dynamics and mitophagy, as well as the appropriate bioenergetics status in mitochondria. Thus, the modulation of polyP could counteract some of the mitochondrial defects observed in aging and neurodegeneration. In the proposed studies, we will couple the use of biophysical, biochemical and imaging-based approaches with the use of systems with different levels of polyP, as well as cellular models where misfolded proteins and mitochondrial dysfunctions are present. Thanks to an outstanding and knowledgeable group of mentors, advisors and collaborators, the mentored phase of the K99/R00 Career Pathway to Independence will provide me with the expertise in aging, animal work, neurosciences and protein homeostasis that I need to conduct a deep study of mitochondrial protein misfolding. This training will complete my extensive background in mitochondrial biology, cell imaging and pharmacology, and will definitely facilitate my transition to independence. The characterization of an innovative quality control pathway on mitochondrial protein homeostasis will help to unravel the mechanism of mitochondrial dysfunctions in aging and neurodegeneration, paving the road to new therapeutic approaches for these conditions.